The parallelization of optical communications using wavelength division multiplexing (WDM) has led to tremendous increases in fiber-optic channel capacity over the past decade. A WDM system usually employs multiple optical signals, each of which is assigned a particular signal wavelength. These optical signals are typically multiplexed to form a group of WDM optical signals that include a mixture of signals from each individual optical signal, transmitted over a single waveguide, and then demultiplexed such that each optical signal wavelength is individually routed to a designated receiver.
WDM technologies can leverage the vast available spectrum of optical channels which exceed many THz. The optical channels may include guided channels such as integrated waveguide and fiber optic channels, and unguided channels, such as free space optical (FSO) channels. Each optical signal may use a manageable amount of bandwidth (e.g., GHz-class) that may, in practice, be generated with available electronics.
Many applications of WDM may benefit from implementations that have small size, low weight, and low power consumption (SWAP). For example, space-based communications, such as those over distances exceeding 1000 km, are usually power-starved, therefore simple low-SWAP receiver (RX) implementations with good sensitivity are desirable since they reduce the burden on the power-starved transmitter (TX) on the other end of the link. In addition, improvements in RX sensitivity may provide more link margin, extend link distances, and enable lower-power TXs, which have an added benefit of reducing nonlinear impairments. Such features are generally attractive to both fiber-optic guided and FSO applications. Furthermore, it would also be beneficial in space-based applications to have reconfigurable WDM receivers that may be used to demultiplex and/or demodulate signals modulated at different wavelengths or using different modulation schemes (e.g., multiple frequency-shift keying or differential phase-shift keying), and support varying data rates. In this case, the receiver in space may accommodate changes of transmitting conditions, link distances, and channel losses, and interoperate with a variety of different TX sources, providing versatility and WDM scalability. These characteristics are particularly useful for space-based FSO applications where launch costs are expensive and the performance and agility may provide significant value.